Plaster, an inexpensive, easy to make product, in great supply, would constitute a quality binding and building material if, when used, it did not exhibit a certain number of serious drawbacks. Its three main drawbacks are a too rapid setting rate, the need to use too great an amount of mixing water and the mediocre mechanical strength of the product.
Actually, one of the main problems encountered in using plaster is the difficulty in obtaining a composition that, while beginning to harden shortly after application, is still able to be worked, i.e., smoothed with a trowel or finished in any other suitable way before setting to give a hard coating. If this finishing time or setting rate is too fast, the plaster acquires a final hardness before having been subjected to the finishing treatment. The influence of this drawback on the economic level is considerable because only small amounts of plaster can be prepared and applied in a necessarily short period. Thus, two objectives are aimed at: a suitable delay of the moment when setting begins (frequently called the flow point) and a sufficiently reduced setting rate (rather long finishing time).
Numerous additives, well known in the prior art, make it possible to achieve the first objective, but the second poses many unsolved problems.
The need to use too high amounts of mixing water also causes concern, particularly because of the very long and, thereby onerous, drying times. Thus, if 90% mixing water is used--from which 20% for crystallization and 16% for normal moisture are deducted--there remains 54% water, or about 10 to 12 kg water per m.sup.2 for a coating 2 cm thick. The resulting mediocre mechanical resistances come in part from the too high amounts of water used for mixing the plaster.
Many attempts have already been made to extend the finishing time, but none of them have given entirely satisfactory results because of secondary effects that quite often are very bothersome. German Pat. No. 742,023 describes the use of nitrilotriacetic acid and its salts, but in practice this additive has to be used with a water-holding agent to improve the workability and consistency of the plaster and at times with a gluing agent to regulate the finishing time.
Hydrolized proteins of animal or vegetable origin, described in U.S. Pat. Nos. 2,207,336; 2,383,252; 2,412,156 and in Canadian Pat. No. 408,094, cause changes in color of the plasters thus modified which go from white to more or less deep yellow, while the mechanical resistances are reduced. Setting retardants, such as phosphoric and citric acids (German Pat. No. 613,105), sodium hexametaphosphate (U.S. Pat. No. 2,216,207), and magnesite (Soviet Pat. No. 64,569) either require a much greater amount of mixing water than pure plaster or cause a reduction of the mechanical properties of the plasters thus modified.
On the other hand, there have already been attempts to reduce the amount of plaster mixing water, although when it is desired both to delay setting of the plaster and to reduce its amount of mixing water, it is necessary to resort to two specific products: a retardant and a fluidizing agent or water reducer. However, their undesirable secondary effects most often are additive, and quite often the fluidizing agent works against the action of the setting retardant; hence there exists the need of putting in a larger dose of setting retardant to obtain the same delaying effect, all of this involving a very considerable increase in the cost of the plaster thus treated.